PHYSIOLOGY, CHEMISTEY AKD PATHOLOGY 453 



fetal life. The fetal membranes of twin sheep are also fused, but there is 

 no vascular anastomosis. The females in these two-sexed twins are normal 

 anatomically and functionally. The same is true of two-sexed twins in 

 cattle if the chorions do not fuse (Fig. 10). 



In mammals the interstitial cells are present in the testes from the 

 very beginning of sex differentiation. In the ovary differentiation occurs 

 later than in the testes because the first generation of ingrowths from the 

 germinal epithelium a true homologue of the seminiferous tubules of 

 the male forms only the medulla, while the cortex is formed from a 

 second generation of ingrowths. The testes of the male of two-sexed 

 twins therefore are present and exert an influence before the ovaries 

 have developed their structure or function. Such a condition, of necessity, 

 brings about suppression of the specific ovarian tissue from the very 

 beginning (Lillie, Chapin). 



Such an explanation of the factors resulting in the formation of the 

 free-martin lends weight to the argument of van der Broek, who contends 

 that the cause of pseudohermaphroditism is an imbalance of gonadal hor- 

 mones between the mother and fetus. 



Heterologous Transplants and Development of the Generative Organs. 

 The influence of the sex hormone upon the generative organs is further 

 show?* by the results following the transplantation of a gonad of one sex 

 into an immature and previously castrated animal of the opposite sex. 

 As previously stated, Steinach, Sand and Moore have been able to change 

 completely the instincts and to a great extent alter the sex characters of 

 rats and guinea pigs by the substitution of a gonad of the opposite sex in 

 pre-adolescent castrates. Further than this, Sand, and later Steinach 

 succeeded in establishing simultaneous transplants of an ovary and 

 testis in the same castrated infantile animal. Such procedures result in 

 the production of somatic hermaphroditism combined with a decided- bi- 

 sexualism of the psychosexual character. 



These and similar observations definitely point to the interstitial cells 

 of the testis as responsible for the deliverance into the blood stream of 

 properties which bring about the proper establishment of the secondary 

 internal generative organs, and indicate that the sexually indifferent 

 somatoplasm is influenced to form sex characteristics in response to the 

 type of the gpnadal hormone acting upon it. 



At puberty modifications of bodily structure occur, which result in the 

 establishment of the secondary sexual characters. This time, there- 

 fore, may be known as the Secondary Differentiated Stage. The advent 

 of this period is ushered in by an enormous proliferation and ac- 

 tivity of the interstitial cells, and ripening of the germ cells (Tandler, 

 Allen, Whitehead, and Marshall). Sex and sex characters are fully 

 established and the active sexual life begins. It is true that nothing new 

 somatically is added to the individual at puberty; however, we 



